Commonly the word “almond” refers to the edible seed of the almond tree (Prunus dulcis). There are several types of almonds including nonpareil, Carmel, California, and Mission Type, produced by varieties of almond trees.
Although almonds are grown across the world, most are produced in the United States. According to the February 2017 USDA report, “Tree Nuts: World Markets and Trade,” global almond production is 1.2 million metric tons of shelled product, of which over 920,000 tons are produced in the United States (https://apps.fas.usda.gov/psdonline/circulars/TreeNuts.pdf).
Profitability of the almond crop is subject to demand and the costs of production. Demand has increased in recent years, and now the yearly production is roughly 130,000 tons more than it was just four years ago. Modern conventional cultivation methods are useful to maximize yield but the economics of almond production is still dependent on the trees' genetics.
Almond tree flowers, as well as other flowers, are comprised of several reproductive parts. The reproductive process of almond trees can be described as when the stamen produces pollen which then germinates once it comes into contact with the stigma. In response to germination, a pollen tube develops extending down from the stigma to the flower's ovule. Pollen moves through the pollen tube to the ovule and the resulting contact between pollen and ovule is fertilization, allowing both seeds and fruits to develop. The effective pollination period (EPP) is the time frame during which pollination can produce fruit most effectively and is determined by the longevity of the ovule and the time needed for the pollen tube to grow from the stigma to the ovule after germination. Although some plants self-fertilize, others such as almond trees require cross-pollination. For almond trees the most effective window for fertilization is within the first two days of a flower opening, and after 3-4 days, the flower cannot be fertilized. Studies have shown that it takes 96 to 120 hours for pollen tubes to grow through the flower to reach the embryo sac (Griggs, W. H.; Iwakiri, B. Pollen tubes growth in almond flowers. California Agriculture, 1975 29(7):4-7) so presumably the sooner pollination occurs after a flower opens, the greater the chance of successful fertilization and fruit set. By this virtue, actions or treatments that increase pollen tube growth rate should expand the effective pollination period (EPP), resulting in increased fertilization and ultimately, an increase in the fruit set.
Brassinosteroids (“BRs”) and gibberellins (“GAs”) are two families of plant growth regulating hormones.
Brassinosteroids are found throughout the plant kingdom and have unique growth promoting activity when applied to plants (Mandava, N. B. Plant growth promoting brassinosteroids. Annual Review of Plant Physiology and Plant Molecular Biology 1988, 39; 23-52). Brassinolide, the first identified brassinosteroid, was first isolated in 1979 from rape (Brassica napus L.) pollen, where it is present in quantities up to 200 parts per billion.
Since the discovery of the chemical structure of brassinolide (“BL” CAS #74174-44-0, FIG. 1), it has been synthesized and analogues have been developed, often from readily available plant sterols. Among the brassinosteroids shown to have similar biological activity as BL are epibrassinolide (“epi BL” CAS #78821-42-8, FIG. 2), homobrassinolide (“HBR” CAS #80483-89-2, FIG. 3), castasterone (“CS” CAS #80736-41-0, FIG. 6), 28-homocastasterone (“HCS” CAS #83509-42-6, FIG. 7). Castasterone and 28-homocastasterone are converted to BL and HBR, respectively, in vivo.
BRs are reported to increase yields and improve stress resistance of several crop plants (Cutler, H. G., et. al. Brassinosteroids: chemistry bioactivity, and applications. ACS symposium Series 474. Washington D.C., 1991; American Chemical Society). Treatments with BRs are effective ways of increasing yield of many crops even in cases of drought, extreme temperatures, and improper soil salinity. For example, in almond trees, BRs have been shown to counteract the slowed pollen tube growth rate caused by lower temperatures (Bernard, D. and Socias, R. I Company. Characterization of Some Self-compatible Almonds. II. Flower Phenology and Morphology. HortScience, 1995, 30(2): 321-324).
Gibberellins were first isolated in 1935 and are known in the art for their involvement in seed germination. Gibberellins are also important modulators of plant gene expression and can stimulate stem and root growth. Gibberellic acid (“GA3” CAS #77-06-5, FIG. 4) promotes growth and elongation of cells and stimulates plant growth when used in small amounts.
Forchlorfenuron (“CPPU,” FIG. 5) is another plant growth regulator which can improve fruit size and fruit set of blueberries, grapes, and kiwi. https://www.epa.gov/site/production/file/2015-04/documents/exhibit_e.pdf
Methods of increasing almond fertilization and increasing almond kernel size can result in a higher return on investment for growers.